Right, but it's (relatively) trivial for police to obtain a warrant to search my phone, all they have to do is convince a (sympathetic) judge they have "probable cause" to believe there's relevant evidence stored there. There is no warrant that can be obtained to force me to answer questions, make statements, or directly hand over evidence (such as the key to a complex cypher, for instance), if I believe that to do so might (rightly or wrongly) incriminate me. The 4th speaks of "unreasonable" -- but the 5th has "nor shall be compelled in any criminal case to be a witness against himself". No mention of "reasonable," just "shall not" and that's it, you can't legally be so compelled (even "good cop, bad cop" interrogation might be seen as coercion in this light, but I don't know of any case that's been upheld in court). And that means that I cannot be forced to give up an unlock code, even though my fingerprint (or a physical dongle) can be used if police can obtain it.